Good advice for short responses, but not for long responses, especially if one wants to respond to specific portions.
I did some testing on that advice, and discovered something:
There appears to be a <return> code for the viewport about three lines beyond the limits of the viewport. The <return> code can be moved down if one starts typing before it. But the closer you get to the <return> code while typing, the synchronization becomes worse and worse.
It appears that when a user types anything in the viewport, the user is not actually typing into a viewport object, but rather typing into the code itself?
Is there any way for the techies in this website to fix this?
It appears to be a matter of removing or revising one simple conditional statement in the code.
As others have noted, this does not seem to happen in other websites.
If one Writes a Response Above the Quotes, it doesn't jump around...But if one writes below 2 or 3 quotes or even 1 long Quote it jumps Around for some reason...... 
Yes, that's what I experience. So I'm going to try what you say and write above the quotes. I wondered why some people did that; now I know. Thanks Stashko.
Selam